1. Field of the Invention
The present invention relates to an electric power steering apparatus for assisting a steering power of a steering wheel by using an electric motor.
2. Description of Related Art
In an electric power steering apparatus, steering torque is detected so as to assist a steering power by using the rotation power of a motor driven in accordance with the detected torque.
FIG. 1 is a schematic block diagram of the configuration of the electric power steering apparatus. The output from a torque sensor 2 provided on a steering shaft (not shown) is inputted to a CPU 1 via an interface 3. The output is subjected to A/D conversion 11 and then phase compensation 12 in the CPU 1. The CPU 1 stores, in its memory, an target current table 13 regarding the relationship between the detected torque and an target current value for driving a motor 5 as shown in FIG. 2, and reads the target current value corresponding to the detected torque from the table 13. The output resulting from the A/D conversion 11 is also subjected to differentiation 18, and addition 14 of the differentiated value and the target current value is performed for compensation of the inertia of the motor 5.
Then, deviation 15 between the result of the addition 14 and a feedback value of the driving current for the motor 5, which is detected by a driving current detection circuit 6 and is subjected to A/D conversion 19 in the CPU 1, is obtained. The CPU 1 then performs PI calculation 16 of the result of the deviation 15 to obtain a control volume, and generates a PWM wave signal 17 and a signal indicating the rotation direction of the motor 5 in accordance with the control volume. These signals are supplied to a drive circuit 4 including an H-type bridge of four switch transistors as well as to the motor 5. Although it is not shown in FIG. 1, speed information detected by a speed sensor is also inputted to the CPU 1, and the target current value corresponding to the detected speed is read from the table 13.
Such an electric power steering apparatus also includes an inhibition circuit 7 for restricting the driving direction of the motor 5 as countermeasure using hardware against runaway of the CPU 1. By the inhibition circuit 7, the motor 5 is inhibited to be driven when the direction of the detected torque and the driving direction of the motor 5 are different from each other, thereby preventing an unforeseen accident from occurring. Actually, the motor 5 is inhibited to be driven also in a dead zone shown in FIG. 2 in which the detected torque is approximately 0. Specifically, when the torque detected by the torque sensor 2 is in a leftward driving inhibition area in FIG. 2, the inhibition circuit 7 generates a leftward driving inhibition signal so as to maintain the potential of a portion in the drive circuit 4 contributing to the leftward driving at a level where the motor 5 is not rotated, thereby allowing the rightward driving alone. Similarly, when the detected torque is in a rightward driving inhibition area, the inhibition circuit 7 generates a rightward driving inhibition signal, thereby allowing the leftward driving alone.
The detected torque, however, is subjected to the addition to the differentiated value in the CPU 1 as described above for the compensation of the inertia of the motor 5. When a car is running at an intermediate speed, the torque detected in moving the steering wheel lightly in one direction is so small that the driving current read from the target current table 13 is 0 in the dead zone, but still the addition 14 of the differentiated value is conducted. In this case, the detected torque is so small that it is in the leftward or rightward driving inhibition area. Therefore, a driving current corresponding to the differentiated value is not supplied, resulting in giving a driver unpreferable steering feeling as if he were rotating a steering wheel with a large flywheel.
Further, when the steering wheel is returned to the initial central position, it is desired to control the direction of the detected torque to be reverse to the driving direction of the motor 5 so that the steering torque in the dead zone be rapidly returned to neutral. In such a case, the detected torque is small, and hence, the table 13 is provided with a hysteresis characteristic as shown with alternate two dots and chain lines in FIG. 2 so as to enable the above-mentioned control. When the inhibition circuit 7 is provided, however, such a control is not conducted. Therefore, it is disadvantageously impossible to drive the motor 5 in the aforementioned manner in returning the steering wheel to the initial position.